The klystron may be regarded as the most highly developed among conventional microwave tubes. It has a wide range of applications, from communication transmission at low power levels to high-energy particle acceleration where tens of megawatts are required. The simplest (though not necessarily the most practical) klystron configuration consists of two cavities separated by a linear drift region. See M. Chodorow and C. Susskind, Fundamentals of Microwave Electronics (McGraw-Hill, New York, 1964), J. W. Gewartowski and H. A. Watson, Principles of Electron Tubes (Von Nostrand, New York, 1965). The input signal is injected into first cavity to provide a velocity modulation of the electrron beam. This velocity modulation, after being carried through the drift region, becomes a density modulation near the second cavity where the output power is extracted. Because of the mutual Coulombic repulsion among the ac space charges, the charge bunching near the output cavity cannot reach the level expected from kinematic (ballistic) considerations. In fact, the efficiency of the klystron depends sensitively on the grouping of electrons near the output cavity. See A. Staprans, E. W. McCume, and J. A. Ruetz, Proc. IEEE 61, 299 (1973).
One approach to compensate for the mutual Coulombic repulsion among the ac space charges has been to develop multicavity klystrons. The effect of each cavity is to reinforce the correct modulation, thus offsetting somewhat the mutual Coulombic repulsion effect. However, the addition of intermediate cavities would make the device bulky and costly. See C. Bastiens, G. Fallon, and M. Simon, Extremely High Power Klystrons for particle accelerations, IEEE Int'l Electron Devices Meeting, Technical Digest, PP. 190-194, Dec. 1982, and Staprans, E. W. Meume and J. A. Ruetz, supra.
The gyrotron-klystron (gyroklystron) has received some attention in the past. However, development of the gyrotron-klystron so far has been quite disappointing and there appears to be little recent activity. It lags far behind the gyrotron oscillators or the gyrotron traveling wave amplifiers. Such a state of affairs is in sharp contrast to conventional microwave sources. See H. R. Jory, Gyro-Device developments and applications. IEEE Int'l Electron Devices Meeting, Technical Digest, PP. 182-185, Dec. 1981.
The most impressive result is a gyroklystron reported by the Soviets to have 70% efficiency. Unfortunately no other information, other than a brief reference to this achievement, is available in the literature about this device. See H. R. Jory, supra. A gyroklystron amplifier at Varian Assoc. in California, produced 50 kW pulsed output at 28 GHz with gain of 40 dB and efficiency of 9%. Calculations predicted a power output of 280 kW with efficiency of 44%. The poor agreement was thought to be caused by velocity spread and space charge effects in the beam. Calculations predict a useful maximum bandwidth for the gyroklystron of about 1%. See H. R. Jory, supra.